The present invention generally relates to stormwater management and, more particularly, to a modular device and system for roof or ground-level stormwater management.
As urban and suburban areas develop and transform once permeable land into impermeable surface, either as buildings, urban infrastructure, roadways, or parking facilities, water from rain and storm events moves much more quickly and directly into municipal storm drains and into natural streams and creeks. In urban areas, storm events may be costly to home and business owners and damaging to the environment. In suburban areas, increased runoff contributes to intensified stream-flow and the eroding of hillsides and creek banks, both of which may destabilize homes and infrastructure and disturb natural habitats. In large storm events, excess runoff may contribute to overflows from combined sewer systems (i.e., storm and sanitary), pollution of rivers with fertilizers, sewage, oil and sediment, destruction of aquatic and riparian habitats, and property damage.
Combined sewer overflows occur when surge events in stormwater create situations in which the mix of stormwater and sewage that normally stays below ground and is routed to a treatment facility instead backs up into streets, overflows into rivers, or even backs up in basement plumbing outlets.
Rivers and streams are polluted as stormwater moves quickly across impermeable surfaces, such as roadways and parking lots, collecting oils and chemicals and washing them directly into rivers. Permeable ground slows the flow of water, which then picks up less sediment and slowly seeps through the ground, which acts as a filter.
Wildlife habitats may be destroyed as the sediments and oils are picked up and washed into streams and rivers. The banks of streams and creeks may also be transformed in high-flow events, disturbing natural wildlife conditions.
Property damage may occur as stormwater runoff overwhelms the municipal infrastructure and water or a mix of water and sewage backs up into streets and basements. The flooding is both damaging and unsanitary. Municipal infrastructure is also at risk, as more stress is placed on aging stormwater systems.
Because of the damaging effects of excess stormwater, municipalities are increasingly focused on developing solutions that rely on both public and private investment to manage stormwater. Faced with either upgrading or replacing their stormwater systems to handle the increased demand (a costly and politically challenging expense) or implementing policies that regulate and mitigate increased non-permeable areas, most municipalities choose the latter.
Local municipalities are also held accountable for the effects of their runoff by the federal government. The Clean Water Act, passed by Congress in 1972, establishes water quality standards for surface water. States enforce these standards by regulating combined storm-sewer overflow points and regulating runoff in areas with separated sewer systems.
Public investment often involves strategies of “greening” cities by increasing plant life along street frontages and within parks. Trees, bushes, and other plant life can remove significant amounts of water from the ground surface.
Private investment in stormwater management is encouraged in primarily two ways: through stormwater fees and through development regulations and incentives.
While municipalities have a range of policies regarding stormwater fees, generally they are implemented in similar ways. Commercial properties may be charged a square-foot cost for all the impermeable square feet located on their property. This encourages them to either build less or to retrofit an existing structure or surface lot to contain more permeable land. Residential property owners may be charged a flat rate based on the average impermeable surface area throughout residential properties in the city. Some municipalities, such as Washington D.C., have stormwater credit markets in which properties owners may receive credits by increasing their stormwater retention and then sell those credits to other owners who may use them to meet minimum requirements.
Stormwater fees may help cover the cost of maintaining the runoff system while also incentivizing property owners to increase their permeable surface area.
Properties being newly developed may be subject to an increasing amount of regulations and municipal scrutiny with regard to stormwater management. For example, in Philadelphia, any new development that disturbs over 15,000 SF of earth triggers Philadelphia Water Department (PWD) regulations. Generally, stormwater regulations require projects to manage the first inch of stormwater runoff on their property onsite. Architects, civil engineers, and their clients can propose a range of solutions to manage the water, but the design must be reviewed and approved by the PWD Stormwater Plan Review office before any permits can be issued for the project's construction.
Developers are also incentivized by many municipal zoning bonuses that can be activated by incorporating stormwater management strategies. When developing a property, the possibility of additional stories, lot coverage or an easing of massing restrictions effectively motivates developers to incorporate green roofs and other stormwater management strategies.
There are currently many methods to manage stormwater onsite. In large suburban areas, retention ponds, landscaped swales, and rain gardens may capture runoff and provide a permeable area with plant life to allow water to deeply infiltrate the earth or be used and eventually transpired by the plant life.
In urban areas, more compact methodologies are generally employed. The most commonly used systems include water cisterns, blue roofs, subsurface detention, and green roofs.
Water cisterns are large storage tanks, located above or below ground, that hold rainwater for reuse or later drainage. These may drain by gravity or may require a pump. Their installation may be inexpensive, though often they are employed underground, requiring an expensive pump to move the water into its new use or the stormwater system. To meet municipal requirements, they often must be combined with filters that must be maintained and inspected. Cisterns located above ground must also be monitored in freeze-thaw temperature cycles. They do not minimize the urban heat island effect.
Blue roofs may provide temporary storage of stormwater on flat roofs by check dams or by restricting the roof drains. Water held during a rain event may then be drained once the storm surge is over. While fairly inexpensive to install, a blue roof tests the waterproofing of a roof by allowing a body of water to sit on the roof, which roof design standards usually seek to avoid. Regular inspections are required of both the roof surface and drains. There are also negatives associated with allowing a still body of water to sit within urban areas, especially as mosquito-borne illness continues to be a growing concern throughout North America.
Subsurface detention methods are underground structures that may be used to temporarily hold and later release stormwater. These may include vaults, stone storage systems, pipe systems or systems of plastic grids. For emergency flooding, in 100 year flood events and the like, many large institutions may repurpose their underground parking facilities to hold stormwater, essentially intentionally flooding a basement (or basements) to mitigate flooding on the floors above. These systems often must be combined with other management strategies to meet water quality requirements and may be costly to install and maintain.
A common stormwater management strategy is a green roof. While there are many variations in green roof design, green roofs generally include a plastic tray with soil and plant life that is placed over a water retention mat and filter fabric along with some form of integrated drainage and water storage. Green roofs may help with stormwater by holding water that would otherwise leave the roof through downspouts for a period of time, either to drain out later or to be used by the plants.
Green roofs may offer other benefits beyond stormwater management, and are often seen as a building amenity. However, green roofs are expensive to install and maintain. Their success is not guaranteed and is dependent on ongoing maintenance, often times involving irrigating the green roof during dry spells. If the plant life dies, many of the benefits of the green roof may disappear, including benefits related to minimizing stormwater, mitigating the effects of urban heat islands, and increasing wildlife.
Another disadvantage for green roofs, from a building owner's perspective, is that if the roof leaks, it may be difficult to discover the source of the leak without tearing out much of the established plant life.
The weight of a green roof may negate it from consideration for retrofits to older buildings. There exist large swaths of urban buildings with flat roofs that were not designed to support the added weight of a green roof.